There is a striking similarity between cognitive functions associated with the left and right hemispheres and the types of mentation reported during sleep. The visuospatial processes of the right hemisphere (RH) resemble the visual hallucinatory processes which tend to occur during Stage 1 REM; the conceptual -verbal processes of the left hemisphere (LH) resemble the "thought-like" mentation which is associated with NREM. The proposed research will test the hypothesis that differences in sleep mentation can be accounted for by shifts in the relative "dominance" of the two hemispheres during sleep. The hypothesis will be tested by obtaining mentation reports when criterion levels of hemispheric "dominance" occur during Stages 1 REM and 2 (REM). Hemispheric dominance will be assessed by measuring bilateral EEG integrated activity. This technique has been shown in a number of studies to reflect asymmetrical hemispheric functioning during waking. It has also yielded differences between REM and NREM in one study (Goldstein et al., 1972) Thus, the technique is well suited to studying hemispheric dominance during sleep. The following procedure will be used: Ss will be run on 4 equals nights. On night 1, criterion values of hemispheric dominance (D equals R-L integrated EEG activity/1/2 (R plus L) integrated EEG activity) will be generated by computing the upper and lower quartile values of D separately for STage 1 REM0and Stage 2 (where upper quartile equals relative RH dominance, and lower quartile equals relative LH dominance). On 3 subsequent nights, Ss will be awakened when criterion values occur. Thus, Ss will be awakened 4 times on each of 3 nights: 1) upper quartile, REM; 2) lower quartile, NREM; 3) upper quartile, NREM; 4) lower quartile, REM. D values will be comouted on-line by a PDP-8 computer which will signal E when criterion values are met. Mentation reports will be rated by "blind" judges for a number of qualities, such as Visual Imagery and Conceptual Thought. Analysis of variance and multiple regression models will be applied to the data Our goal is to provide a physiological model which can both account for variance unexplained by precent models and offer a mechanism for the varieties of mentation during